Revisiting primordial black hole capture into neutron stars

Primordial black holes (PBH), produced through a variety of processes in the early universe, could fill galactic halos accounting for a fraction or the totality of the dark matter. In particular, PBH with substellar masses could be captured by stars, start to swallow their material, and eventually turn them into BH, hence originating a peculiar new type of stellar catastrophic event. Here we revisit this process in the most favorable case of PBH capture by neutron stars. We detail a number of novel features in the capture phase, during the settling within the star and mass growth of the PBH, and illustrate some phenomenological consequences. In particular, we point out that in the subsonic regime the PBH drag takes the form of a Bondi accretion. As a result, the onset of the final transmutation of the NS into a stellar sized BH is expected with the PBH seed in slight off-center position. We also compute the gravitational wave energy-loss and signals associated to different phases of the PBH-stellar interaction. In particular, the emission associated to the accretion phase is periodic with a few kHz frequency; in the rare case of a nearby Galactic event and for light PBH, it could constitute a warning of the forthcoming transmutation.

Figure 1

Sound speed (thick line) and PBH speed (thin line) as a function of the radius for the NS reference profiles considered.

Figure 2

Contribution of the different processes of energy-losses to capture: dynamical friction (Dyn), accretion (Acc), surface waves (Sw) and gravitational waves (Gw, magnified by a factor 50), as a function of the impact parameter ratio $b/b_c$, for the benchmark $v_i={10}^{-3}$.

Figure 3

Evolution of the function $\mathcal{C}$ (black-dashed line) of Eq. (27) as a function of $X$ defined in Eq. (28). The sole contribution of GW capture is displayed in blue and the difference with the total is shown with a dashed gray line.

Figure 4

Examples of PBH trajectories (black lines) for impact parameters $\le b_c$. The star is displayed in red, the construction of this trajectory (black) from two hyperboles (green and blue) and one ellipse (dashed orange) is shown with dashed lines. The radial scale is in units of $R_{\star }$.

Figure 5

Evolution of the gravitational strain for the trajectories shown Fig. 4. $h_c$ is in units of ${10}^{-25}$, assuming $v_i={10}^{-3}$, $d=1\mathrm{kpc}$ and $m={10}^{25}\mathrm{g}$.